Gas springs of this type have numerous different damping devices, with which different damping characteristics and hence different extension and retraction speeds are attained. In hatchbacks or trunk lids of motor vehicles, further mechanical clamping devices or hydropneumatic locking devices have been provided so as to assure that in the extended position of the gas spring, that is, with the trunk lid or hatchback raised, or with the hood up, the lid or hood could not fall unintentionally if the pressure in the gas spring should drop.
Gas springs, and even damped gas springs, are usually not used for articles that are intended to be in a position of repose at any level, because counterbalancing is extraordinarily difficult. For such purposes, so-called oil supports are used, which are complicated in structure and in particular require absolutely tight valve plates (see U.S. Pat. No. 4,156,523 and German patent application DE-OS No. 25 55 062).
In this context, it is already known from German utility model DE-GM 85 02 121 to provide a mechanical clamping device for a gas spring, this device being embodied by an arbitrarily infinitely adjustable friction brake. To this end, a friction element of which the pressing force is infinitely variable and which acts elastically upon the piston rod is provided.
A replicable damping can be attained only with difficulty with this kind of clamping device, because of the relatively small surface area of the piston rod. Furthermore, there is the danger that the surface of the piston rod may be damaged, which would cause the gas spring to lose its tight seal.